The present invention relates to solar energy collection and in particular to coatings for producing selective absorber surfaces.
A wide variety of solar collectors are presently available, which collectors may be classified into three major types. The first and essentially simplest type of collector is that of the non-tracking flat plate type, which consists essentially of a flat absorber panel enclosed in a collector housing having a window over the absorber, which structure is oriented towards the sun and usually remains fixed in position. This type of structure may be movable for adjustments during the solar year but for the most part it normally remains stationary during any particular period of collection, e.g., day, month, season or year. Another type of collector is that using an evacuated tube or tubes which surrounds an absorber surface. This type of collector normally is in a fixed position for a particular collection period as mentioned above but it has the advantage of having an evacuated space for reducing convection and conduction losses from the absorber surface. Yet another type of solar collector is the concentrating type which may include a flat plate or evacuated tubular collector located at a concentrating zone or focus of appropriate concentrating apparatus. The collector may be of the fixed type which has a relatively low concentration ratio, a wide acceptance angle for relatively long periods of the solar day and may require little or no tracking. On the other hand, the most sophisticated and expensive of the concentrating collectors are those having a relatively narrow acceptance angle with high concentration and which consequently require tracking over the entire period of the solar day.
Each of the aforementioned types of collectors serves a specific market. For example the non-tracking flat plate collector serves the domestic hot water and heating market, whereas the evacuated tubular type, whether fixed or partially tracking, may be used to produce hot water, heat and air conditioning. Finally, the highly concentrating tracking collectors may be utilized for the production of high temperature working fluid for power generation. It should be realized that each of the systems has constraints which are rather severe and that substantial cost reduction must be realized in order to economically justify a solar collector installation as an alternative to other sources.
To this end a spectrally selective absorber coating is an essential component of most efficient collector designs. Existing coatings, although quite efficient, as evidenced by high absorptivity .alpha. and low emissivity .epsilon., are very expensive. Exotic and expensive materials such as indium, gold and silver compounds are sometimes used to produce spectrally selective absorber surfaces. Considering the square footage requirements of absorber surfaces which are necessary to compete with just the domestic hot water and heating alternatives, such exotic and expensive materials are not economically attractive.
It has been found that some films of metal oxides, when combined in proper juxtaposition, provide useful selective absorber surfaces, which surfaces are valuable improvements in the selective absorber technology, since they can be produced in large quantities at reasonable costs.
The type of surface contemplated by the present invention is one in which the absorptivity .alpha. is measured in the visible and near visible wavelengths of solar radiation from about 0.2 micrometers to about 2 micrometers, and the emissivity .epsilon. is measured in a range of the infrared and near infrared domain from about 2 micrometers to about 20 micrometers.
It is known that certain coatings for absorbers exhibit selectivity, in that they are opaque to incident solar radiation but on the other hand are transparent to infrared. For example, black chrome on copper has been found to absorb in the range of visible radiation with an absorptivity of 0.9 and the polished copper substrate "looks through" the black coating to reflect infrared radiation.
A rather comprehensive summary of some of the problems and phenomenon discovered in connection with selected coatings is discussed by Seraphin in an article entitled Converting Solar Radiation to Heat:Challenges to Optical Material Science, published in Optical Science Center Newsletter 10, No. 1, 1976, University of Arizona, Tuscon. Absorber-reflector tandems are discussed in that article wherein two basic configurations are described as follows: (1) heat mirrors, wherein the reflector intercepts the sunlight first, and are characterized by highly doped semiconductors such as indium oxide, tin oxide or cadmium stannate, which are highly reflective in the thermal infrared but are transparent to the incident solar energy; (2) absorber reflector configuration, wherein the absorber is transparent to longer wavelengths so that the reflector can "look through" and suppress the emittance in the thermal infrared. Other types of absorbers are discussed such as semiconductor absorbers and those having various controlled refractive indexes.
Nozic et al. in U.S. Pat. No. 3,987,781 discusses the use of a cadmium stannate electrically conductive coating which suppresses infrared radiation. Gillory in U.S. Pat. No. 3,981,293 discusses a figure of merit for absorption and reflection in a solar collector for a heat mirror window. Mochel, on the other hand, discusses in his U.S. Pat. No. 3,202,054 the use of multiple coatings for reflecting infrared radiation to suppress the heat buildup in a building due to incident sunlight. Similarly Dates in U.S. Pat. No. 3,473,944 describes a heat reflecting glass panel which reflects a substantial portion of radiation throughout the visible spectrum and also absorbs a certain amount of radiation so as to both prevent glare and permit the viewing of objects therethrough without color distortion. None of the aforementioned references show the arrangement of specially formulated coatings for a solar absorber as described herein.
It has been found that tin, indium and certain iron oxide coatings when combined in a tandem arrangement can act as both an absorber and as an infrared mirror, which coatings are relatively easy to apply to a substrate such as glass. On the one hand, tin oxide films have been used for infrared mirrors, but have not as yet been formed as effective absorbers. Iron oxide, however, has been found to be a reasonably good absorber but a poor infrared reflector.
The present invention seeks to utilize the materials set forth above in a manner which is an improvement over the described prior arrangements, since the materials serve a dual function of exhibiting high absorptivity in the visible range and good infrared reflectivity in the desired infrared range.